Re: [RFC PATCH 3/9] cxl/mem: Add a driver for the type-3 mailbox

[Dan Williams]

On Thu, Dec 3, 2020 at 11:22 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Tue, Nov 17, 2020 at 6:50 AM Jonathan Cameron
> <Jonathan.Cameron@xxxxxxxxxx> wrote:
> >
> > On Tue, 10 Nov 2020 21:43:50 -0800
> > Ben Widawsky <ben.widawsky@xxxxxxxxx> wrote:
> >
> > > From: Dan Williams <dan.j.williams@xxxxxxxxx>
> > >
> > > The CXL.mem protocol allows a device to act as a provider of "System
> > > RAM" and/or "Persistent Memory" that is fully coherent as if the memory
> > > was attached to the typical CPU memory controller.
> > >
> > > The memory range exported by the device may optionally be described by
> > > the platform firmware memory map, or by infrastructure like LIBNVDIMM to
> > > provision persistent memory capacity from one, or more, CXL.mem devices.
> > >
> > > A pre-requisite for Linux-managed memory-capacity provisioning is this
> > > cxl_mem driver that can speak the "type-3 mailbox" protocol.
> > >
> > > For now just land the driver boiler-plate and fill it in with
> > > functionality in subsequent commits.
> > >
> > > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > Signed-off-by: Ben Widawsky <ben.widawsky@xxxxxxxxx>
> >
> > I've tried to avoid repeats, so mostly this is me moaning about naming!
> >
> > Jonathan
> >
> > > ---
> > >  drivers/cxl/Kconfig  | 20 +++++++++++
> > >  drivers/cxl/Makefile |  2 ++
> > >  drivers/cxl/mem.c    | 82 ++++++++++++++++++++++++++++++++++++++++++++
> > >  drivers/cxl/pci.h    | 15 ++++++++
> > >  4 files changed, 119 insertions(+)
> > >  create mode 100644 drivers/cxl/mem.c
> > >  create mode 100644 drivers/cxl/pci.h
> > >
> > > diff --git a/drivers/cxl/Kconfig b/drivers/cxl/Kconfig
> > > index dd724bd364df..15548f5c77ff 100644
> > > --- a/drivers/cxl/Kconfig
> > > +++ b/drivers/cxl/Kconfig
> > > @@ -27,4 +27,24 @@ config CXL_ACPI
> > >         resources described by the CEDT (CXL Early Discovery Table)
> > >
> > >         Say 'y' to enable CXL (Compute Express Link) drivers.
> > > +
> > > +config CXL_MEM
> > > +        tristate "CXL.mem Device Support"
> > > +        depends on PCI && CXL_BUS_PROVIDER != n
> > > +        default m if CXL_BUS_PROVIDER
> > > +        help
> > > +          The CXL.mem protocol allows a device to act as a provider of
> > > +          "System RAM" and/or "Persistent Memory" that is fully coherent
> > > +          as if the memory was attached to the typical CPU memory
> > > +          controller.
> > > +
> > > +          Say 'y/m' to enable a driver named "cxl_mem.ko" that will attach
> > > +          to CXL.mem devices for configuration, provisioning, and health
> > > +          monitoring, the so called "type-3 mailbox". Note, this driver
> > > +          is required for dynamic provisioning of CXL.mem attached
> > > +          memory, a pre-requisite for persistent memory support, but
> > > +          devices that provide volatile memory may be fully described by
> > > +          existing platform firmware memory enumeration.
> > > +
> > > +          If unsure say 'n'.
> > >  endif
> > > diff --git a/drivers/cxl/Makefile b/drivers/cxl/Makefile
> > > index d38cd34a2582..97fdffb00f2d 100644
> > > --- a/drivers/cxl/Makefile
> > > +++ b/drivers/cxl/Makefile
> > > @@ -1,5 +1,7 @@
> > >  # SPDX-License-Identifier: GPL-2.0
> > >  obj-$(CONFIG_CXL_ACPI) += cxl_acpi.o
> > > +obj-$(CONFIG_CXL_MEM) += cxl_mem.o
> > >
> > >  ccflags-y += -DDEFAULT_SYMBOL_NAMESPACE=CXL
> > >  cxl_acpi-y := acpi.o
> > > +cxl_mem-y := mem.o
> > > diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> > > new file mode 100644
> > > index 000000000000..aa7d881fa47b
> > > --- /dev/null
> > > +++ b/drivers/cxl/mem.c
> > > @@ -0,0 +1,82 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +// Copyright(c) 2020 Intel Corporation. All rights reserved.
> > > +#include <linux/module.h>
> > > +#include <linux/pci.h>
> > > +#include <linux/io.h>
> > > +#include "acpi.h"
> > > +#include "pci.h"
> > > +
> > > +struct cxl_mem {
> > > +     void __iomem *regs;
> > > +};
> > > +
> > > +static int cxl_mem_dvsec(struct pci_dev *pdev, int dvsec)
> > > +{
> > > +     int pos;
> > > +
> > > +     pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_DVSEC);
> > > +     if (!pos)
> > > +             return 0;
> > > +
> > > +     while (pos) {
> > > +             u16 vendor, id;
> > > +
> > > +             pci_read_config_word(pdev, pos + PCI_DVSEC_VENDOR_OFFSET, &vendor);
> > > +             pci_read_config_word(pdev, pos + PCI_DVSEC_ID_OFFSET, &id);
> > > +             if (vendor == PCI_DVSEC_VENDOR_CXL && dvsec == id)
> > > +                     return pos;
> > > +
> > > +             pos = pci_find_next_ext_capability(pdev, pos, PCI_EXT_CAP_ID_DVSEC);
> >
> > This is good generic code and wouldn't cause much backport effort (even if needed
> > to bring in a local copy), so perhaps make it a generic function and move to
> > core PCI code?
> >
> > Mind you I guess that can happen the 'second' time someone wants to find a DVSEC.
> >
> > > +     }
> > > +
> > > +     return 0;
> > > +}
> > > +
> > > +static int cxl_mem_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> > > +{
> > > +     struct device *dev = &pdev->dev;
> > > +     struct cxl_mem *cxlm;
> > > +     int rc, regloc;
> > > +
> > > +     rc = cxl_bus_prepared(pdev);
> > > +     if (rc != 0) {
> > > +             dev_err(dev, "failed to acquire interface\n");
> > > +             return rc;
> > > +     }
> > > +
> > > +     regloc = cxl_mem_dvsec(pdev, PCI_DVSEC_ID_CXL_REGLOC);
> > > +     if (!regloc) {
> > > +             dev_err(dev, "register location dvsec not found\n");
> > > +             return -ENXIO;
> > > +     }
> > > +
> > > +     cxlm = devm_kzalloc(dev, sizeof(*cxlm), GFP_KERNEL);
> > > +     if (!cxlm)
> > > +             return -ENOMEM;
> > > +
> > > +     return 0;
> > > +}
> > > +
> > > +static void cxl_mem_remove(struct pci_dev *pdev)
> > > +{
> > > +}
> >
> > I'd bring this in only when needed in later patch.
> >
> > > +
> > > +static const struct pci_device_id cxl_mem_pci_tbl[] = {
> > > +     /* PCI class code for CXL.mem Type-3 Devices */
> > > +     { PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID,
> > > +       PCI_CLASS_MEMORY_CXL, 0xffffff, 0 },
> > > +     { /* terminate list */ },
> > > +};
> > > +MODULE_DEVICE_TABLE(pci, cxl_mem_pci_tbl);
> > > +
> > > +static struct pci_driver cxl_mem_driver = {
> > > +     .name                   = KBUILD_MODNAME,
> > > +     .id_table               = cxl_mem_pci_tbl,
> > > +     .probe                  = cxl_mem_probe,
> > > +     .remove                 = cxl_mem_remove,
> > > +};
> > > +
> > > +MODULE_LICENSE("GPL v2");
> > > +MODULE_AUTHOR("Intel Corporation");
> > > +module_pci_driver(cxl_mem_driver);
> > > +MODULE_IMPORT_NS(CXL);
> > > diff --git a/drivers/cxl/pci.h b/drivers/cxl/pci.h
> > > new file mode 100644
> > > index 000000000000..beb03921e6da
> > > --- /dev/null
> > > +++ b/drivers/cxl/pci.h
> > > @@ -0,0 +1,15 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +// Copyright(c) 2020 Intel Corporation. All rights reserved.
> > > +#ifndef __CXL_PCI_H__
> > > +#define __CXL_PCI_H__
> > > +
> > > +#define PCI_CLASS_MEMORY_CXL 0x050210
> > > +
> > > +#define PCI_EXT_CAP_ID_DVSEC 0x23
> > > +#define PCI_DVSEC_VENDOR_CXL 0x1E98
> >
> > Hmm. The magic question of what to call a vendor ID that isn't a vendor
> > ID but just a magic number that talks like a duck and quacks like a duck
> > (for anyone wondering what I'm talking about, there is a nice bit of legal
> > boilerplate on this in the CXL spec)
> >
> > This name is definitely not accurate however.
> >
> > PCI_UNIQUE_VALUE_CXL maybe?  It is used for other things than DVSEC (VDMs etc),
> > though possibly this is the only software visible use.
>
> Finally working my way back through this review to make the changes.
> If 0x1E98 becomes visible to software somewhere else then this can
> become something like the following:
>
> #define PCI_DVSEC_VENDOR_CXL PCI_UNIQUE_VALUE_CXL
>
> ...or whatever the generic name is, but this field per the
> specification is the DVSEC-vendor-id and calling it
> PCI_UNIQUE_VALUE_CXL does not have any basis in the spec.
>
> I will rename it though to:
>
> PCI_DVSEC_VENDOR_ID_CXL
>
> ...since include/linux/pci_ids.h includes the _ID_ part.
>
> >
> >
> > > +#define PCI_DVSEC_VENDOR_OFFSET      0x4
> > > +#define PCI_DVSEC_ID_OFFSET  0x8
> >
> > Put a line break here perhaps and maybe a spec reference to where to find
> > the various DVSEC IDs.
>
> Ok.
>
> >
> > > +#define PCI_DVSEC_ID_CXL     0x0
> >
> > That's definitely a confusing name as well.
>
> Yeah, should be PCI_DVSEC_DEVICE_ID_CXL

Actually, no, the spec calls this the "DVSEC id" so PCI_DVSEC_ID_CXL
seems suitable to me. This is from:

Table 126. PCI Express DVSEC Register Settings for CXL Device

In the CXL 2.0 Specification.